Various types of bridge have a deck supported by tensile supports from towers, or similar structures, erected at, or intermediate, the ends of the bridge. In the case of a suspension bridge the tensile supports are typically vertical cables, rods or chains interconnecting each longitudinal side of the deck to a corresponding catenary suspended between the towers. A cable-stayed bridge also comprises a deck supported by tensile supports, usually in the form of rods or cables, extending from the longitudinal sides of the deck directly to the towers.
It is well known from the Tacoma bridge disaster in 1940 that a suspension bridge can suffer dramatic structural failure due to fluttering instability in a sustained modest wind loading which caused a resonant oscillation of the deck which built up progressively until destruction occurred. The problems associated with wind loading of suspension bridges, and indeed all bridges comprising a deck supported by tensile supports, become much more severe as the span of the deck increases. With a very long span, for instance that proposed for the Straights of the Messina, the wind loading along the span can vary substantially and can promote substantial asymmetric pitching and heaving of the deck. Since the Tacoma bridge disaster, various proposals have been made to address this problem. For instance, in European Patent 0233528, it has been proposed that a suspension bridge, comprising a suspension structure formed of cantenary wires and vertical stays and a substantially rigid planar deck structure hung onto the suspension structure, could be stabilised by aerodynamic elements which are shaped like aerofoils and are rigidly fixed to the bridge structure to control the action of the wind on the structure, the aerodynamic elements consisting of wing control surfaces which have a symmetrical profile and an aerodynamic positive or negative lifting reaction together with a flutter speed considerably higher than the flutter speed proper to the bridge structure, the wing surfaces being fixed just under the lateral edges of the deck structure of the bridge, with their plane of symmetry inclined in respect of the horizontal plane, the bridge structure and the wing control surfaces interacting dynamically in order to shift the flutter speed of the whole at least above the top speed of the wind expected in the bridge area.
Instead of using aerofoils rigidly fixed to the bridge structure, International Patent Application PCT/GB93/01862 (Publication Number WO 94/05862) teaches that a bridge deck can be made less stiff than the decks of existing bridges by using flaps, or ailerons, provided at the lateral edges of the bridge deck, the flaps or ailerons being pivoted from the bridge deck for articulation between extended and retracted positions, and being computer controlled to regulate the forces on the deck in response to wind loading.
International Patent Application PCT/DK-93/00058 (Publication Number WO 93/16232) teaches a system for counteracting wind induced oscillations in the bridge girder on long cable supported bridges, wherein a plurality of control faces are arranged substantially symmetrically about the longitudinal axis of the bridge and are adapted to utilise the energy of the wind in response to the movement of the bridge girder for reducing said movement, the control faces being divided into sections in the longitudinal direction of the bridge, and a plurality of detectors are provided for measuring the movements of the bridge girder, and a local control unit is associated with each control face section and is adapted to control the control face section in question in response to information from one or more of the detectors. These detectors are arranged to measure the movements or accelerations of the bridge at the point concerned and to transmit a signal to a control unit, such as a computer, which uses an algorithm to apply a signal to a servo pump controlling a hydraulic cylinder to rotate the associated control face section. In this manner, each control face section can be adjusted continuously in response to the movements of the bridge girder at the point in question as measured by the detectors which are in the form of accelerometers. This invention essentially requires the provision of a complex electronic system incorporating a significant number of accelerometers connected by extensive wiring along the bridge girder to the computers, and an associated hydraulic system for driving the control faces.
From WO 93/16232 and these prior art documents it is known for a bridge to comprise a deck supported by tensile supports, and aerofoil stabilisers pivoted about respective axes generally longitudinal of the deck for articulation to a position to improve stability of the deck.
It is also known from these documents to provide a method of stabilising a bridge having a deck supported by tensile supports including mounting aerofoil stabilisers about respective axes generally longitudinal of the deck.